Please cc Andy on this stuff. I can't imagine patching entry_64.S at this point without cc'ing him. *Surely* you didn't even bother to run get_maintainer.pl on this.
> @@ -214,6 +215,11 @@ > .macro SWITCH_TO_KERNEL_CR3 scratch_reg:req > ALTERNATIVE "jmp .Lend_\@", "", X86_FEATURE_PTI > mov %cr3, \scratch_reg > + > + /* if we're already on the kernel PGD, we don't switch */ > + testq $(PTI_SWITCH_PGTABLES_MASK), \scratch_reg > + jz .Lend_\@ > + > ADJUST_KERNEL_CR3 \scratch_reg > mov \scratch_reg, %cr3 > .Lend_\@: This is an optimization that we can do generally without your feature. Actually, it would be a welcome bit of benchmarking if you could see if just this hunk helps your workload. You touched on it in the description, but this is a *very* clever way to do what you need without needing to look at the task flag at user->kernel entry which also happens to be a place you don't have task_struct mapped. It *greatly* simplifies what this would have to do otherwise. That needs calling out specifically though.
